I think you are right. That would explain why so many of the Planet Defender clones are of the tower defence type rather than having the player directly control one spaceship. I have just found out there is a template for the player controlled ship version of this game available for Unity, which explains why there are several identical versions on Itch from different users.
I was asking because I have made my own version in Godot styled to look like it is an early 80's arcade game (filled polygons rather than vector graphics though) as that is where the game seemed to fit best. I wanted to credit the original in my game's page but I will probably have to forget that. Anyway thanks for your help!